Cause
The campaign is no longer a draft: scheduling, opening or cancelling it took it out of editing, or the action requires a draft.
What to do
Check the state of the campaign. For another edition create a campaign with another external_id.
application/problem+json
{
"type": "https://masnet.com/developers/errors/campaign_not_draft",
"title": "Campaign is not a draft",
"status": 409,
"detail": "...",
"code": "campaign_not_draft",
"correlation_id": "..."
}